There were two particularly significant changes among the North American mammalian carnivores that took place around this time. One was the extinction of the bear-dogs, a group with a long history on the continent that was, by this time, typified by relatively large animals that would have looked more like bears than dogs (although they were neither). Over in Europe and Asia, the bear-dogs survived for rather longer, although even there they struggled... but in North America, the changes were too extreme for them to cope with, and they died out as the Late Miocene dawned.
This, as it turned out, was good news for the real dogs and bears, but also for some rather smaller carnivores, especially the mustelids. While the giant honey badger Eomellivora, which is better known from Europe, did also reach North America, it does not seem to have been common there, and most of its local relatives were closer to what we'd see today.
Plesiogulo, for example, looked much like a wolverine, and seems to have entered North America from Asia around 7 million years ago. It lived in woodland areas, with one collection of fossils found in Kansas apparently representing a mother with three of her young. Despite its resemblance to modern wolverines, it probably isn't a direct ancestor, but rather a close relative that left no descendants of its own.
Other mustelids of the time included animals resembling martens, weasels, and what's probably an early relative of the American badger, Chamitataxus, an animal not directly related to the 'true' badgers of Eurasia, and that likely has a long - and almost entirely mysterious - evolutionary history in North America. Otters were also present, and include Enhydritherium, an animal that was, for a long time, only known from coastal deposits. This, together with some of its other features, suggests that it resembled, and was perhaps related to, modern sea otters, rather than the more widespread freshwater species.
While it seems to have reached North America from Eurasia, its marine habits mean that it didn't necessarily do so via any Bering land bridge - it could equally well have crossed the North Atlantic. However, this does rely on it being as fully aquatic as we suspect. While its teeth suggest that it ate both fish and shellfish, which doesn't really narrow things down, detailed analysis of the skeleton indicates that it would not have been as fully aquatic as modern sea otters, which sleep at sea, rather than on riverbanks or shorelines. Confirmation of this came in 2017 when a fossil was discovered in river deposits 200 km (125 miles) inland in Mexico, which also helped to explain how the species could have lived in both California and Florida, which don't exactly share a nearby coastline.
Towards the end of the Middle Miocene, the large predatory dog Osbornodon had been replaced by more advanced counterparts, most notably Aelurodon. By the Late Miocene, however, that had begun to give way to even more advanced species, which were among the top predators in North America at that time.
These dogs, like Aelurodon, belonged to the "borophagine" lineage, a group often associated with hyper-carnivory and sometimes compared with modern hyenas. That was by no means true of many of the early species, but by this time they were certainly earning that comparison. An exception was Carpocyon, represented by four species, ranging from coyote to wolf-sized, but having a diet that, while it certainly contained meat, was likely more omnivorous than those of its counterparts. It mostly lived in what is now the American Southwest, although a few fossils have been found as far north as Oregon.
Far more impressive, however, was the mighty Epicyon, the largest dog ever to have lived. With similar bodily proportions to a hyena, but closer in size to a black bear, some estimates suggest a body weight, for the largest species, of around 170 kg (375 lbs). Although none were exactly small, not all were quite so large; in many parts of America, two species lived side-by-side, apparently avoiding competition by one being noticeably larger than the other, and so presumably tackling different prey. As the Late Miocene wore on, individuals became ever larger, but when the epoch came to a close, they rapidly went extinct, perhaps unable to cope with the changes in their prey that occurred around that time.
Even more specialised was Borophagus, the last and, from a certain perspective, most highly evolved of the borophagine lineage that bears its name. It was far smaller than Epicyon, from which it may have evolved, although still comparable to some of the largest breeds of domestic dog today (which, of course, are much larger than wild wolves). The real specialisation, however, came in its teeth, which were unusually large and solid for an animal of its size, adapted for crushing up the bones of its prey to extract the maximum nutrition. Unlike wolves, it probably wasn't especially fast, being an ambush predator and scavenger, much like the spotted hyenas of today.
We don't just know this by inference from the shape, size, and structure of its teeth, significant though those are. Just last year, some fossilised droppings were discovered in California, and identified as having been left by Borophagus. Not only did they contain a significant amount of crushed-up bone, but the way that they had been placed suggests that the dogs used them to scent-mark their territory. The fact that the animals the crushed bones belonged often seem to have been much larger than the dogs would have been also suggests that they hunted in packs, although probably not ones quite so cooperative as modern wolves.
Borophagus lasted longer than Epicyon did, narrowly making it into the subsequent, Pliocene, epoch. But, when it did eventually die out, it left no descendants, and the borophagines vanished for good. Two other dogs that lived at the same time had, in the longer term, more success. One of these was Eucyon, a fairly unremarkable looking animal that resembled a coyote in appearance and size, and probably also in habits. Its significance comes from the fact that, as the Miocene drew to a close, it became one of the first dogs to leave the Americas, heading east into Asia, eventually reaching as far as Europe and Africa. Descended from a different canid lineage than the borophagines, it was probably the ancestor of all of today's wolves, coyotes, and jackals.
The second kind of "dog" that first appeared in North America at this time is one that we don't normally call a dog at all. Foxes first evolved around 9 million years ago, and their descendants have changed so little since that these early forms are typically placed in the modern genus, Vulpes. They first left the continent at, or shortly after, the end of the Miocene, giving rise to the many species of fox in the Old World today. We know less about the ancestors of the South American foxes, but they too, seem to have lived around what was then the southern coastal region of North America, the Panama isthmus not yet having formed.
At the beginning of this post, I said that there were two major shake-ups to the mix of carnivores present in North America as the Middle Miocene drew to a close. One was the extinction of the bear-dogs, but the other occurred somewhat earlier. This was the end of the "Cat Gap", as cat-like animals re-entered the continent after an absence of seven million years...
[Photo by "Ghedoghedo" from Wikimedia Commons.]